The present invention relates to a hydraulically boosted hydraulic braking system comprising:                a master cylinder comprising a thrust chamber and on command receiving pressurized brake fluid from an accumulator via an electrically operated valve controlled by a computer on the basis of the signal supplied by the brake pedal sensor,        an electrical power source powering at least the electrically operated valve, the computer and the sensor.        
In the event of failure of the electrical supply afforded by the on-board circuit, which is typically an alternator and a 12-volt lead-acid battery, the brakes cannot operate without boosting because the electro-hydraulic boosting disappears: the electrically operated valve that controls the supply to the control chamber of the master cylinder cannot be actuated which means that even if the pressure accumulator contains a reserve of pressurized fluid, this reserve cannot act on the master cylinder to bring about a braking action.
Emergency operation can be afforded only mechanically via a control rod driven by the brake pedal which then operates the primary piston of the master cylinder directly.
When operating the brakes in this failure situation the driver has to supply a thrust that corresponds to the sum of the thrust needed to control the piston of the master cylinder (or the primary piston in the case of a tandem master cylinder) and of the reaction applied by any brake simulator that might be present.
The braking operation becomes awkward if not difficult in some cases especially for a driver who is not strong enough to brake without the assistance of a brake booster.
This situation becomes all the more paradoxical when, in the event of the failure of the vehicle electrical power system, the pressure accumulator may still be full and still be capable of providing hydraulic control of the master cylinder for a few braking actions.